The present invention is directed to a process for producing solid particulate dialkylesters from their corresponding dianhydrides. In particular, the present invention is directed to the production of the dimethylester of benzophenonetetracarboxylic acid (BTDE) from benzophenonetetracarboxylic dianhydride (BTDA). BTDE has specific utility in the formation of polyimide resins used to produce reinforced high temperature fabrics.
The polyimide resins to which the present invention relates are noted for their high thermal and oxidative stability, high strength at elevated temperatures, and many other outstanding physical and chemical properties useful in high temperature applications such as jet engine compression components, for example blades, vanes, air seals, air splitters and engine casing parts. The method for preparing high molecular weight addition type polyimides is described in U.S. Pat. No. 3,745,149 herein incorporated by reference.
In general the method for preparing the polyimides includes forming a mixture of a polyfunctional amine, a polyfunctional ester and an end-capping agent. In particular, esters of tetracarboxylic acid produced from corresponding dianhydrides are shown as useful in the production of the polyimide resin. While the end-capping agent and the polyfunctional amine are available in 100% solid component, the polyfunctional ester, typically BTDE, has always been prepared and sued from an alcohol solution, typically methanol. This procedure for preparation of BTDE generates detrimental quantities of the tri-or tetra- ester over time due to the presence of excess alcohol. Therefore, because of the storage instability, the BTDE solution had to be prepared on site and/or had to be used within a certain period of time to provide the resulting polyimide resin with its proper properties.
The present invention is directed to providing a method of manufacturing BTDE in solid form. The advantages of the invention are that the BTDE prepared by the process of the present invention contains substantially less undesirable bi-coproducts because excess alcohol is not present. Furthermore, the BTDE because it is in solid form is stable and can be stored for indefinite period. Finally, because the solid BTDE does not require the presence of methanol other polyfunctional amines which have not been able to be used because of their insolubility in methanol can now be used to develop different polyimide resins which will have new properties.